Togekiss. With the addition of this evolution togepi's line transformed from being cute into a juggernaut. Even with super-effective moves you're not smashing through its defenses in one turn. Then he has so many options to heal himself or even get one of your allies straight by making a wish. Status moves won't even work thanks to heal bell.
His ability serene grace is just unfair... a strong stab move in air slash bumped up to a whopping 60% chance for flinch means you never may be able to even get started. Those doubled odds make everything look good on this winged little creature from shadow ball to ancient power.
You can pretty much teach him to kill any typing you want to because his list of coverage moves are just ridiculous. Not the least of which includes a strong, never-missing, fighting move in aura sphere, perfectly tailored to special attackers like himself.
If he chooses to set himself up with say a nasty plot or calm mind then the apocalypse is coming, just say your prayers and bid the world good bye. There's not much you can do about it because all this cuddly critter has to do is yawn at you before hand, you'll either be asleep or forced to flee the field, either way its free turn for togekiss and the end is coming :p